Biological Corridor 3

Biological Corridor 3 is a designated wildlife corridor in the Sarpang district of southern Bhutan, situated in the Sarpang subregion that borders India's Assam state to the south. This corridor occupies a critically important position in Bhutan's national biological corridor network, connecting the protected subtropical forests of southern Bhutan with habitats extending into the broader Assam-Bengal-Bhutan landscape, which represents one of the most significant transboundary conservation areas in South Asia. Sarpang district encompasses a transition zone between the subtropical foothills of the Himalayan piedmont and the lowland forests of the Indo-Gangetic plain, with the biological corridor preserving ecological linkages across this gradient. The corridor is part of a system designed to allow wildlife to move between Royal Manas National Park to the east and other protected areas of southern Bhutan, supporting populations of large mammals including tigers and elephants that range across international boundaries. The broader landscape of which this corridor forms a part is recognized as one of the most important wildlife areas in the eastern Himalayas.

Biological Corridor 3 in Sarpang supports some of the most significant large mammal populations in Bhutan, taking advantage of the corridor's subtropical lowland and foothill forest habitats that are among the most biodiverse in the entire Himalayan region. Bengal tigers use the corridor as movement habitat between populations centered in Royal Manas National Park and adjacent areas, and camera trap surveys have documented tiger activity in the Sarpang corridor zone. Asian elephants are a prominent feature of the wildlife in the Sarpang region, with herds that range across the international border between Bhutan and India's Assam state, making the corridor critical for maintaining connectivity for this landscape-scale species. One-horned rhinoceroses, whose main population exists in India's Kaziranga National Park to the south, have been documented in southern Bhutan, and the corridor habitats of Sarpang may facilitate occasional movements of this endangered species. Gaur, clouded leopards, sloth bears, and wild pigs represent other significant mammals using the corridor. The bird diversity of the Sarpang region, encompassing species from the subtropical and tropical zones, is exceptionally high.

The vegetation of Biological Corridor 3 reflects the subtropical character of the Sarpang district, with tropical and subtropical broadleaf forest communities dominating the lower elevations of the corridor and transitioning to warm temperate forests at higher altitudes. Tropical and subtropical moist broadleaf forest in the lowest sections supports exceptionally high plant diversity, including large emergent trees, dense mid-canopy species, diverse woody climbers, and rich epiphyte communities of orchids, ferns, and bromeliads-like plants. Sal trees dominate much of the subtropical forest in the lower Sarpang area, as they do across a broad belt of the Himalayan foothills from Bhutan westward through Nepal, India, and Bangladesh. The ground flora of these subtropical forests is highly diverse, with dense herbaceous understory layers in areas with sufficient light and moisture. Tall grasses, including species characteristic of the elephant grasslands that are a distinctive feature of the broader Brahmaputra floodplain ecosystem, occur along the southern margins of the corridor where forest grades into more open habitats. The corridor's floral diversity is a direct reflection of its position at the junction of subtropical and tropical vegetation zones.

Sarpang district in southern Bhutan occupies the transition from the Himalayan mountain system to the Brahmaputra floodplain, encompassing the geologically distinctive Himalayan foothill zone known as the Siwalik Hills or Churia Range. The Siwaliks are the youngest and outermost range of the Himalayan system, composed of relatively soft sedimentary rocks including sandstone, conglomerate, and mudstone eroded from the rising main Himalayan ranges and deposited in the foreland basin during the past several million years. These Siwalik rocks are highly erodible, and the rapid erosion of the Himalayan foothills in Sarpang district contributes significant sediment loads to the rivers flowing south into India's Assam plain. Active tectonics in the region means that the Himalayan front in Sarpang is seismically active, with significant earthquakes occurring periodically along the Main Frontal Thrust fault system. The gravel and alluvial fans deposited by rivers emerging from the mountains create the distinctive terai landscape that provides the ecological setting for much of the corridor's lowland wildlife habitat. River dynamics in the region are highly active, with channels shifting and flooding during the monsoon season.

Biological Corridor 3 in Sarpang experiences a humid subtropical climate in its lower reaches, with conditions becoming progressively cooler and more temperate at higher elevations within the corridor zone. The southern Bhutanese foothills receive among the highest rainfall in Bhutan, with the Himalayan front forcing moist monsoon air rapidly upward and generating intense orographic precipitation. Annual rainfall in parts of Sarpang district can exceed 3,000 millimetres (118 inches), concentrated in the monsoon season from June through September. This extreme rainfall creates the lush subtropical forest vegetation characteristic of the southern Bhutanese corridor and drives the high river flows that periodically flood lowland areas. Temperatures in the lower corridor are warm throughout the year, with summer maximums exceeding 35 degrees Celsius (95 degrees Fahrenheit) in the lowlands and winter minimums rarely dropping below 10 degrees Celsius (50 degrees Fahrenheit). The combination of high heat and humidity during the monsoon season makes the lower corridor environment challenging for visitors. The cooler months from October through March provide the most comfortable conditions and coincide with lower river levels and improved visibility in the forests.

The Sarpang district and its forests have been inhabited by diverse communities for centuries, including indigenous groups such as the Lhops and various farming communities that settled the fertile foothills and valleys of the Himalayan piedmont. The forests of southern Bhutan were historically regarded as wild and challenging frontier zones, with malaria historically limiting permanent settlement in the lowest foothill areas where the biological corridor now lies. The consolidation of Bhutanese territory in the southern foothills occurred over several centuries as the kingdom expanded its control over the diverse communities of the terai and foothill zones. Tensions along the southern Bhutanese border with India have at various times influenced the human landscape of Sarpang district, and the corridor passes through a region that has seen significant demographic change in the modern era. Traditional use of forest resources including timber, non-timber forest products, and hunting was historically practiced by communities adjacent to what is now the corridor zone, and managing the transition from open-access forest use to formal conservation has been an ongoing process.

Biological Corridor 3 was formally designated as part of Bhutan's national biological corridors system, designed to complement the network of protected national parks and wildlife sanctuaries by maintaining ecological connectivity between them. The Sarpang corridor's importance was highlighted by wildlife surveys that documented the significant large mammal populations in this zone and its critical role in connecting the Bhutanese protected area network with the broader transboundary conservation landscape extending into India's Assam state. The corridor design in Sarpang required careful consideration of the existing human settlements and agricultural areas in the district, as the southern Bhutanese foothills have a more complex pattern of human land use than the high mountain zones where other corridors operate. International conservation organizations including the World Wildlife Fund have been active in the Sarpang and Royal Manas landscape, providing technical support for wildlife monitoring, anti-poaching efforts, and community engagement programs in the corridor zone. The establishment of the corridor has been integrated with broader conservation programs targeting the transboundary landscape that spans Bhutan and India.

Sarpang district and its biological corridor are not primary destinations on the main Bhutanese tourist circuit, but the region offers distinctive wildlife watching opportunities in the context of the broader Royal Manas National Park landscape. The proximity of the corridor to Royal Manas National Park, which is one of Bhutan's premier wildlife destinations, means that wildlife experiences in the Sarpang zone can complement visits to the national park. Elephant observation, either from elephant-back safari or from vehicles, has historically been part of the experience in southern Bhutanese protected areas. Bird watching in the subtropical forests of Sarpang is highly rewarding, with the species diversity of the Indo-Gangetic plain and Himalayan foothills converging in this zone. The forest environments of the corridor represent a dramatically different ecological experience from the high mountain landscapes that many visitors associate with Bhutan, offering a complementary perspective on the country's remarkable biodiversity. Access to the Sarpang area from central Bhutan requires road travel through mountain terrain.

Sarpang district is accessible by road from the main Bhutanese highway network, though the journey from Thimphu or Paro involves significant travel through mountain terrain. The district headquarters of Sarpang town provides basic facilities for visitors including accommodation and food. Travel to the biological corridor zone requires compliance with Bhutan's tourism regulations requiring booking through licensed tour operators and payment of the Sustainable Development Fee for international visitors. The Royal Manas National Park adjacent to the Sarpang corridor area has developed some visitor facilities including ranger stations and accommodation to support wildlife tourism, and visits to the corridor zone are best arranged in conjunction with a park visit. The monsoon season from June through September makes travel in the southern foothills difficult due to heavy rainfall, flooded roads, and high leeches, and this period should generally be avoided for visitor access. The best conditions for visiting the Sarpang area are from November through March when temperatures are moderate, roads are passable, and wildlife is more visible in the drier forest understory.

Biological Corridor 3 in Sarpang faces conservation challenges that are among the most complex in Bhutan's corridor network, given the combination of significant wildlife populations, dense human land use in parts of the district, and proximity to the India border. Human-elephant conflict is a serious issue in the Sarpang area, as elephant herds that move through the corridor cause significant crop damage to farming communities along the corridor margins. This conflict represents one of the most pressing conservation-development tensions in southern Bhutan, requiring management approaches that compensate farmers for losses and create space for elephant movement without undermining livelihoods. Anti-poaching enforcement in the southern foothills is challenging given the terrain and the proximity to markets across the Indian border. Habitat loss and degradation in the Indian portion of the broader landscape affects the conservation effectiveness of the Bhutanese corridor, highlighting the importance of transboundary coordination with Indian counterpart agencies. Climate change is expected to shift the distribution of subtropical forest species and alter monsoon patterns in ways that could change the corridor's ecological character over coming decades.